Industrial Instrumentation

Industrial Instrumentation
Dr. –Ing. Naveed Ramzan PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan aaaa 1

Pressure Sensors “In any given plant, the number of pressure gauges used is probably larger than all other instruments put together” Most liquid and all gaseous materials in the process industries are contained within closed vessels. For the safety of plant personnel and protection of the vessel, pressure in the vessel is controlled. In addition, pressured is controlled because it influences key process operations like vapor-liquid equilibrium, chemical reaction rate, and fluid flow. 2

Pressure Measurement Pressure = Force / Area Pressure can be used inferentially to measure other variables such as Flow and Level Pressure plays a major role in determining the Boiling Point of Liquids Fluids exerts pressure on the containing vessel equally and in all directions Instruments used to measure fluid pressure range from very simple to very complex. Some techniques require many moving parts and other none at all. aaaa 3

Pressure Measurement Instruments used to measure fluid pressure range from very simple to very complex. Some techniques require many moving parts and other none at all. aaaa 4

Mechanical Methods Electrical Methods Pressure Measurement Methods
Instruments used to measure fluid pressure range from very simple to very complex. Some techniques require many moving parts and other none at all. aaaa 5

Elastic pressure transducers Manometer method
Pressure Measurement Methods Elastic pressure transducers Manometer method Vacuum Pressure Measurement Electric pressure transducers Pressure measurement by balancing forces produced on a known area by a measured force Instruments used to measure fluid pressure range from very simple to very complex. Some techniques require many moving parts and other none at all. aaaa 6

Bourdon tube pressure gauge Diaphragm pressure transducers Bellows
Elastic Pressure Transducers Bourdon tube pressure gauge Diaphragm pressure transducers Bellows Uses flexible element as sensor. As pressure changed ,the flexible element moved, and this motion was used to rotate a pointer in front of dail.

Bourdon tubes are generally are of three types; C-type Helical type
Bourdon Tube Pressure Gauge Bourdon tubes are generally are of three types; C-type Helical type Spiral type Perhaps the most common device around today is the pressure gauage which utilizes a bourdon tube as its sensing elements. ---Bourdon : A bourdon tube is a curved, hollow tube with the process pressure applied to the fluid in the tube. The pressure in the tube causes the tube to deform or uncoil. The pressure can be determined from the mechanical displacement of the pointer connected to the Bourdon tube. Typical shapes for the tube are “C” (normally for local display), spiral and helical. 8 8

Bourdon Tube Pressure Gauge
Perhaps the most common device around today is the pressure gauage which utilizes a bourdon tube as its sensing elements. ---Bourdon : A bourdon tube is a curved, hollow tube with the process pressure applied to the fluid in the tube. The pressure in the tube causes the tube to deform or uncoil. The pressure can be determined from the mechanical displacement of the pointer connected to the Bourdon tube. Typical shapes for the tube are “C” (normally for local display), spiral and helical. 9 9

Diaphragm are popular because they required less space
Diaphragm and Bellows Pressure Gauge Diaphragm are popular because they required less space and the motion they produce is sufficient for operating electronic transducers Diaphragm : A diaphragm is typically constructed of two flexible disks, and when a pressure is applied to one face of the diaphragm, the position of the disk face changes due to deformation. The position can be related to pressure. aaaa 10

They are used to measure gauge pressures over very low ranges.
Diaphragm Pressure Gauge They are used to measure gauge pressures over very low ranges. Two types of diaphragm pressure gauges are: Metallic diaphragms gauge (brass or bronze) 2. Slack diaphragms gauge (Rubber) Diaphragm : A diaphragm is typically constructed of two flexible disks, and when a pressure is applied to one face of the diaphragm, the position of the disk face changes due to deformation. The position can be related to pressure. aaaa 11

More sensitive than bourdon type gauge.
Bellows More sensitive than bourdon type gauge. Used to measure low pressures Brass, phosphor bronze, stainless steel, beryllium copper etc. Bellows : A bellows is a closed vessel with sides that can expand and contract, like an accordion. The position of the bellows without pressure can be determined by the bellows itself or a spring. The pressure is applied to the face of the bellows, and its deformation and its position depend upon the pressure. A bellows elastic element is a convoluted unit that expands and contracts axially with changes in pressure. The pressure to be measured can be applied to either the outside or the inside of the bellows; in practice, most bellows measuring. Simple Bellows Elements Bellows elastic elements are made of brass, phosphor bronze, stainless steel, beryllium-copper, or other metal suitable for the intended service of the gauge. Motion of the element (bellows) is transmitted by suitable linkage and gears to a dial pointer. Most bellows gauges are spring-loaded—that is, a spring opposes the bellows and thus prevents full expansion of the bellows. Limiting the expansion of the bellows in this way protects the bellows and prolongs its life. Because of the elasticity in both the bellows and the spring in a spring-loaded bellows element, the relationship between the applied pressure and bellows movement is linear.

Electric Pressure Transducers
Mechanical Pressure Transducers to Pneumatic Pressure Transmitters Electric Pressure Transmitters Electrical pressure transducers consists of three elements ---Pressure transducers used in industry today take on a rather typical appearance. A round, tubular stainless steel body with a pipe fitting on one end and a cable coming out of the other end. Stainless is used because of its high strength and resistance to corrosion. What is inside? Starting at the pipe threaded end, the opening or port has a stainless steel diaphragm inside that protects the sensor element from the media being measured (i.e. water). As we continue our journey through the transducer we come to the other side of the diaphragm where one side of the sensor element is. The actual element is a strain gauge; that is, a resistive element whose resistance changes with the amount of strain placed on it. This variable resistor forms one leg of a bridge circuit. The other side of the strain element is the reference port that the measuring port is compared to. All transducers have two sides; sometimes the other side has its own pressure connection and the device is called a differential pressure transducer. The two voltage out points from the bridge circuit are fed to an amplifier that changes the very small voltage into a 0-5V signal or most commonly to a 4-20 mA signal. This signal is fed out the cable (sometimes along with a vent tube) which finishes our voyage. PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

Why Electrical Pressure Transducers?
Electric Pressure Transducers Why Electrical Pressure Transducers? Transmission requirements for remote display as electric signal transmission can be through cable or cordless. Electric signals give quicker responses and high accuracy in digital measurements. The linearity property of the electric signal produced to pressure applied favors simplicity. They can be used for extreme pressure applications, i.e. high vacuum and pressure measurements. EPTs are immune to hysteresis, shock and mechanical vibrations. 4/6/2017 14 Group Se7en

Electric Pressure Transducers
Pressure sensing element such as a bellow , a diaphragm or a bourdon tube Primary conversion element e.g. resistance or voltage Secondary conversion element Electrical pressure transducers consists of three elements ---Pressure transducers used in industry today take on a rather typical appearance. A round, tubular stainless steel body with a pipe fitting on one end and a cable coming out of the other end. Stainless is used because of its high strength and resistance to corrosion. What is inside? Starting at the pipe threaded end, the opening or port has a stainless steel diaphragm inside that protects the sensor element from the media being measured (i.e. water). As we continue our journey through the transducer we come to the other side of the diaphragm where one side of the sensor element is. The actual element is a strain gauge; that is, a resistive element whose resistance changes with the amount of strain placed on it. This variable resistor forms one leg of a bridge circuit. The other side of the strain element is the reference port that the measuring port is compared to. All transducers have two sides; sometimes the other side has its own pressure connection and the device is called a differential pressure transducer. The two voltage out points from the bridge circuit are fed to an amplifier that changes the very small voltage into a 0-5V signal or most commonly to a 4-20 mA signal. This signal is fed out the cable (sometimes along with a vent tube) which finishes our voyage. PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

Strain gauge pressure transducers Capacitive pressure transducers
Types of Electric Pressure Transducers Strain gauge pressure transducers Capacitive pressure transducers Potentiometer pressure transducers Resonant Wire pressure transducers Piezeoelectric pressure transducers PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

Strain Gauge Pressure Transducer
A strain gauge is a passive type resistance pressure transducer whose electrical resistance changes when it is stretched or compressed The wire filament is attached to a structure under strain and the resistance in the strained wire is measured A pressure transducer contains a diaphragm which is deformed by the pressure which can cause a strain gauge to stretch or compress. This deformation of the strain gauge causes the variation in length and cross sectional area due to which its resistance changes. PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan aaaa 17

Construction and Working A pressure transducer contains a diaphragm which is deformed by the pressure which can cause a strain gauge to stretch or compress. This deformation of the strain gauge causes the variation in length and cross sectional area due to which its resistance changes. aaaa 18

-- Strain gauge pressure transducers are used for narrow pressure span and for differential pressure measurments -- Available for pressure ranges as low as 3 inches of water to as high as 200,000 psig -- Inaccuracy ranges from 0.1 % of span to 0.25 % of full scale A pressure transducer contains a diaphragm which is deformed by the pressure which can cause a strain gauge to stretch or compress. This deformation of the strain gauge causes the variation in length and cross sectional area due to which its resistance changes. PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan aaaa 19

Capacitive Pressure Transducer
C=ε0 εr A/d Where, C = the capacitance of a capacitor in farad A = area of each plate in m2 d = distance between two plates in m εr= dielectric constant ε0 = 8.854*10^-12 farad/m2 Thus, capacitance can be varied by changing distance between the plates, area of the plate or value of the dielectric medium between the plates. Any change in these factors cause change in capacitance. A capacitive pressure transducer including a pair of electrically insulative elastic diaphragms disposed adjacent to each other and bonded together in a spaced apart relationship to form a sealed cavity, a conductive layer applied to the inside surface of each of the diaphragms and a small absolute pressure provided in the cavity. This small absolute pressure cavity essentially reduces the effects of the negative temperature coefficient of the modulus of elasticity of the diaphragms. In capacitive transducers, pressure is utilized to vary any of the above mentioned factors which will cause change in capacitance and that is a measureable by any suitable electric bridge circuit and is proportional to the pressure. PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

The sensing diaphragm and capacitor form a differential variable separation capacitor. When the two input pressures are equal the diaphragm is positioned centrally and the capacitance are equal. A difference in the two input pressure causes displacement of the sensing diaphragm and is sensed as a difference between the two capacitances A capacitive pressure transducer including a pair of electrically insulative elastic diaphragms disposed adjacent to each other and bonded together in a spaced apart relationship to form a sealed cavity, a conductive layer applied to the inside surface of each of the diaphragms and a small absolute pressure provided in the cavity. This small absolute pressure cavity essentially reduces the effects of the negative temperature coefficient of the modulus of elasticity of the diaphragms. PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

-- Originally developed for use in low vacuum research -- Wide rangeability from high vacuum in the micron range to 10,000 psig -- Differential pressure as low as 0.01 inch can be readable -- Accurate within 0.1 % of reading or 0.01 % of full scale -- More Corrosion resistant A capacitive pressure transducer including a pair of electrically insulative elastic diaphragms disposed adjacent to each other and bonded together in a spaced apart relationship to form a sealed cavity, a conductive layer applied to the inside surface of each of the diaphragms and a small absolute pressure provided in the cavity. This small absolute pressure cavity essentially reduces the effects of the negative temperature coefficient of the modulus of elasticity of the diaphragms. PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

Potentiometer Pressure Transducer
The device consists of a precision potentiometer whose wiper alarm is mechanically linked to bourdon tube or bellow. The movement of wiper alarm across the potentiometer converts the mechanically detected sensor deflection into a resistance measurement using a Wheatstone bridge circuit. --- 5 – psig PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

Potentiometer Pressure Transducer
-- Extremely small and installed in very tight quarters such inside the housing of 4.5 in dial pressure gauge -- Provide strong output so no need of additional amplifier -- Range 5 to 10,000 psig -- Accurate within 0.5 % and 1 % of full scale --- 5 – psig PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

Resonant Wire Pressure Transducer
A wire is griped by a static member at one end and by the sensing diaphragm at the other. An oscillatory circuit causes the wire to oscillate at its resonant frequency. A change in process pressure changes the wire tension which in turn changes the resonant frequency of the wire. A digital counter circuit detects the shift. --- 5 – psig PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

Resonant Wire Pressure Transducer
-- Used for low differential pressure applications -- Generates inherently digital signal -- Sensitive to shock and variation -- Range : From Absolute pressure mm Hg Up to Differential pressure in Water or Gauge pressure psig -- Accuracy 0.1 % of Calibrated Spam --- 5 – psig PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

Piezoelectric Pressure Transducer
When a pressure , force or acceleration is applied to a quartz crystal , a charge is developed across the crystal that is proportional to the force applied --- 5 – psig PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

Piezoelectric Pressure Transducer
-- Signals generated by crystals decays rapidly so unsuitable for static force or pressure measurements -- measure rapidly changing pressure resulting from blasts, explosions or pulsation pressures -- Range : 5,000 to 10,000 psi -- Rugged construction, small size and high speed --- 5 – psig PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

Where and How have EPTs failed?
EPTs require a constant supply of electricity for them to function. They do not come with built-in power supply. High performance comes at a cost. Installation of auxiliary display modules and electrical circuitry increases capital cost. Physical properties, like temperature, which can affect electrical constants may affect the consistency of EPTs. For this reason, temperature compensation is always required with EPTs. Some electrical phenomena, like piezolectric energy, have limited applicability. This limits their use in industry. Electricity exposes personnel to potential hazards. 4/6/2017 29 Group Se7en

INDUCTIVE/RELUCTIVE PRESSURE TRANSDUCERS
Home Work INDUCTIVE/RELUCTIVE PRESSURE TRANSDUCERS --- 5 – psig PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

INDUCTIVE/RELUCTIVE TRANSDUCERS:
High Pressure and Vacuum Measurement -- Synthetic Diamond Manufacturing industry: Normal reaction pressure = 100,000 psig Some fiber and Plastic extruders operates at 10,000 psig -- But in average plant pressure exceeding 1000 psig are considered high INDUCTIVE/RELUCTIVE TRANSDUCERS: --- 5 – psig PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

High Pressure and Vacuum Measurement
High pressure designs -- Can detect pressure up to 10,000 psig and operate up to 8000 degree F -- The pressure of the output air signal follows the process pressure in inverse ratio to the areas of the two diaphragms. If the diaphragm area ratio is 200:1, a 1,000-psig increase in process pressure will raise the air output signal by 5 psig. --- 5 – psig PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

High Pressure and Vacuum Measurement High pressure designs
-- May include as many as twenty coils -- can measure pressures well in excess of 10,000 psig -- standard element material is heavy-duty stainless steel -- measurement error is around 1% of span -- Suitable for fluctuating pressure service --- 5 – psig PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

High pressure designs -- primary standards in calibrating high-pressure sensors -- Typical inaccuracy of an industrial deadweight tester is 1 part in 1,000 or 0.1%. --- 5 – psig PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

Very High pressure The bulk modulus cell consists of a hollow cylindrical steel probe closed at the inner end with a projecting stem on the outer end . When exposed to a process pressure, the probe is compressed, the probe tip is moved to the right by the isotropic contraction, and the stem moves further outward. This stem motion is then converted into a pressure reading. detect pressures up to 200,000 psig with 1% to 2% full span error --- 5 – psig PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

--- 5 – psig PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

In general, for high vacuum services , either cold cathode or Bayard-Alpert hot cathode gauges are suitable. Neither is particularly accurate or stable, and both require frequent calibration. -- For vacuums in the millitorr range the capacitance manometer is a good choice. For intermediate vacuum Applications capacitance manometers are the best in terms of performance, but are also the most expensive. The lowest priced gauge is the thermocouple type, but its error is the greatest. Digital Pirani gauges can represent a good compromise solution, with accuracy between that of capacitance and thermocouple sensors. For low vacuums (higher pressures) between atmospheric and 10-2 torr, Bourdon tubes, bellows, active strain gages, and capacitance sensors are all suitable --- 5 – psig PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

-- A basic manometer can consist of a reservoir filled with a liquid and a vertical tube . -When detecting vacuums, the top of the column is sealed evacuated. -- A manometer without a reservoir is simply a U-shaped tube, with one leg sealed and evacuated and the other connected to the unknown process pressure -- The difference in the two column heights indicates the process vacuum. -- An inclined manometer can consist of a well and transparent tube mounted at an angle. A small change in vacuum pressure will cause a relatively large movement of the liquid. --Manometers are simple, low cost, and can detect vacuums down to 1 millitorr. --- 5 – psig PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

A capacitance sensor operates by measuring the change in electrical capacitance that results from the movement of a sensing diaphragm relative to some fixed capacitance electrodes --- 5 – psig Accuracy is typically 0.25 to 0.5% of reading. Thin diaphragms can measure down to 10-5 torr, while thicker diaphragms can measure in the low vacuum to atmospheric range. PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

McLeod Gauge: (Originally invented in 1878),
High Pressure and Vacuum Measurement McLeod Gauge: (Originally invented in 1878), -- The McLeod gauge measures the pressure of gases by compressing a known volume with a fixed pressure. The new volume is then a measure of the initial absolute pressure. -- The McLeod gauge has been used until recently for calibrating other gauges. -- It covers the vacuum range between 1 and 10-6 torr. --- 5 – psig PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

Molecular Momentum: -- This vacuum gauge is operated with a rotor that spins at a constant speed. Gas molecules in the process sample come in contact with the rotor and are propelled into the restrained cylinder. The force of impact drives the cylinder to a distance proportional to the energy transferred, which is a measure of the number of gas molecules in that space. -- The full scale of the instrument depends on the gas being measured. -- The detector has to be calibrated for each application. --- 5 – psig PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

Pirani: High Pressure and Vacuum Measurement Thermal Designs:
The thermal conductivity of a gas changes with its pressure in the vacuum range. If an element heated by a constant power source is placed in a gas, the resulting surface temperature of the element will be a function of the surrounding vacuum. Because the sensor is an electrically heated wire, thermal vacuum sensors are often called hot wire gauges. Typically, hot wire gauges can be used to measure down to 10-3 mm Hg. Pirani: In this design, a sensor wire is heated electrically and the pressure of the gas is determined by measuring the current needed to keep the wire at a constant temperature --- 5 – psig PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

Ionization Gauges --- 5 – psig PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

Summary of Pressure Sensor Characteristics
PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

Discussion & Questions?
Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan

Discussion Examples Example 1

Differential Pressure Cell (DP Cell)
It is a device that measures the differential pressure between two inputs. Depending on what class the DP-cell is, it will give you feedback with a current signal. Normal in Europe is 4-20 mA, where 4 is lowest and 20 is highest. Example: You have a pressure container where you connect the H-side (high) of the DP-cell. The L-side (low) you leave in the atmosphere (open air). The DP-cell will tell you the pressure in the pressure container relative to atmosphere pressure. PEC – UET Lahore Dr. Shahid Naveed April 2017 / Dr. –Ing Naveed Ramzan aaaa 53

Industrial Instrumentation

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